We work on optical simulation where we use not ideal beam expander. Not ideal means that for beam expander designed for single mode (M^2=1), the output beam has M^2 >1 (M^2 = M squared) In our system we want to use beam expander with multimode laser beam. The beam expander is not ideal (for example M^2 after beam expander is 5 instead of 1). What will be M^2 of the output beam if the M^2 of the input beam is 30: 30+5 OR 30*5 ? Of course M^2 is not well defined parameter, what is important to us is to understand the phenomena in this case. I hope I am in right place :-) Thanks!